Experimental study of kerosene supersonic combustion with pilot hydrogen and fuel additive under low flight mach conditions

Reliable ignition and efficient combustion are critical for low flight Mach starting stage of dual-mode scramjet. In this paper, liquid kerosene ignition and combustion with pilot hydrogen and fuel additive were experimentally studied in a model supersonic combustor with clean air inflow, under the simulated conditions of flight Mach 3.8–4.0. The selected composite additive is mainly composed of about 20%-vol low-carbon small-molecular unsaturated alkanes and about 80%-vol diethylmethoxyborane. It is found that, under the low-temperature supersonic conditions, either RP-3 kerosene or with additives can all be ignited by the pilot hydrogen flame of least ERH = 0.065–0.076, only fuel additive effects seem to be still not enough for auto-ignition. Self-sustaining flame stabilization and combustion were realized with the tandem dual-cavity flame-holders, an appropriate amount of hydrogen addition can significantly promote the liquid kerosene combustion heat release without inlet disturbing. Enhancement of kerosene supersonic combustion and faster/higher temperature rises with the fuel additive of certain concentration were experimentally validated, and the positive effects showed more significant at a lower equivalence ratio of ERK = 0.43, in which the combustion enhancement induced mode transition from transonic to subsonic is found due to the 20%-vol additive. The additive effects might be greatly depended on the concentration in a nonlinear way when reducing concentration from 20%-vol to 10%-vol, a small concentration of 10%-vol additive did not produce an obvious enhancement effect. A small amount of hydrogen addition or an appropriate proportion of additive can improve the combustor performance, while too high total equivalence ratios or large amount of hydrogen addition tend to lower combustion efficiency.